Industrial processes such as molding and lay up of composite materials, thermoforming, injection molding and reaction injection molding require tools having a shape specific to the article to be made. For example, a composite article can be formed in a mold having a shape corresponding to the shape of the desired article by laying up fibers and a matrix composition such as an epoxy or other polymeric material on the surface of the mold and curing the polymer composition. In some cases, the fibers and composition are held between two mating mold parts so that the fibers and composition are squeezed between the surfaces of the mold parts. In reaction injection molding, two or more mating mold parts are brought together to form a substantially closed cavity and a reactive polymer composition is placed within the cavity and cured to form a shape corresponding to the shape of the cavity. Tools for forming small parts are often machined from solid metals or cast using conventional techniques. These techniques are impractical in the case of very large molds, such as those having dimensions of about a meter (about three feet) or more.
There has been an ever-increasing need for large molds in numerous industries. For example, in the aerospace industry, the increasing prevalence of composite structural materials in airframes has lead to a substantial need for practical large molds. These molds must meet demanding conditions in use. Composite parts used in airframes must meet exacting standards for fit and finish and often incorporate complex curved surfaces. Also, many useful materials such as carbon-fiber reinforced graphite composites must be molded at relatively high temperatures. Molds formed from alloys having low coefficients of thermal expansion such as nickel alloys are preferred for bonding these materials. The cost of machining very large molds from solid blocks of these materials is prohibitive.
As described in greater detail in commonly assigned U.S. Pat. No. 5,817,267 (“the '267 patent”), the disclosure of which is hereby incorporated by reference herein, molds and other tools of essentially unlimited dimensions may be formed from a wide variety of metals, including low-expansion nickel and iron alloys by a thermal spraying process. As described in certain embodiments of the '267 patent, a shell have a working surface with a desired shape can be formed by providing a matrix having the desired shape and spraying droplets of molten metal using a thermal spray gun such as a plasma spray gun or arc spray gun onto the matrix. Such spraying can be used to build up the metal to a substantial thickness, typically about one-quarter inch (6 mm) or more. During the deposition process, the spray gun is moved relative to the matrix so that the spray gun passes back and forth over the surface of the matrix in a movement direction and so that the spray gun shifts in a step direction transverse to the movement direction between passes. The metal is ejected from the gun in a spray direction. The gun is turned between passes so as to change the spray direction between passes. Thus, during at least some successive passes, metal is deposited on the same region of the matrix from two different spray directions in a “crisscross” pattern. The resulting shells have substantial strength and good conformity with the matrix to provide a faithful reproduction of the matrix shape. Although the '267 patent is not limited by any theory of operation, it is believed that deposition of the metal in different spray directions can produce an interwoven pattern of metal droplets and/or metal grains in the deposited shell, and that this produces a stronger, generally better shell.
As also described in the '267 patent, the shell can be strengthened by applying a backing material such as a polymeric reinforcement on the surface of the shell opposite from the working surface. Typically, such reinforcement can include additional structural elements such as metallic members, and may also include elements such as cooling and heating tubes for conducting a fluid to heat or cool the mold in service. Molds of this type provide practical, low-expansion large molds and accordingly have been widely adopted in diverse industries including the aerospace industry.
Other spray-metal mold making processes have also been proposed. Typically, these involve formation of molds from a relatively low-melting, low-strength materials such as zinc.
Despite the success of the molds and processes disclosed in the '267 patent, still further improvement would be desirable. In particular, there has been a need for improved ways of mounting and supporting the shells formed by thermal spraying. The mounting and reinforcing structure should support the shell at many locations on the shell. The need for such support is particularly great in the case of very large molds, where the weight of the shell, the weight of the materials placed in the shell during a molding process, and the pressure applied to the molding composition during the molding process all can cause deformation of the mold. The process to fabricate the support should be simple and economical and should not cause deformation or destruction of the shell. The support should be relatively light to facilitate handling of the mold during use and during shipment. For example, in some composite and thermal set molding processes, a mold is charged with the composition to be molded at a lay-up or filling station and then moved into a curing station such as a large autoclave or oven. Minimizing the weight of the support minimizes the difficulties in handling the filled mold during these steps. Moreover, the processes to provide and attach the support should not unduly complicate the spraying process used to fabricate the shell. All of these factors, taken together, have presented a significant engineering challenge.
Further enhancement in the thermal properties of sprayed molds also would be desirable. In various molding processes heat is transferred between the molding composition inside the mold and heating or cooling elements such as fluid conduits, electrical resistance heaters or the like in thermal contact with the shell. Such heat transfer can be used either to heat or cool the molding composition as required by the particular process. In some cases, local heat transfer problems limit the production rate and the part quality. For example, parts having diverse geometric features such as curves, corners and the like, and parts having relatively thin sections and relatively thick sections, do not cure uniformly. A relatively thick section of the part may be only partially cured when a relatively thin section has already been fully cured. This limits the rate at which parts can be formed in the mold and can cause product defects such as warpage. It would be desirable to provide sprayed shell molds which can help to alleviate these problems.